In recent years, it has been reported that a current drive drastically increases in multi-gate pMOSFETs with uniaxially compressive strain in (110) surface on an SGOI substrate (T. Irisawa et al., IEEE Trans. Electron Devices, 53, 2809 [2006]). For the SGOI multi-gate MOSFETs, an SGOI substrate should be prepared which can be got as following process. First, a SiGe layer is formed on an SOI substrate by epitaxial growth. Next, the SiGe/SOI substrate is oxidized in the oxygen atmosphere. This oxidation results in selective consumption of Si with regard to Ge, which is called as Ge condensation, and Ge composition increases as oxidation proceeds. Thereby an SGOI substrate of desired Ge composition can be formed. In the Ge condensation method, however, it has the following two problems.
The first problem is that an epitaxially grown SiGe layer cannot avoid being compressively strained because it has a larger lattice constant than that of SOI layer which supports the SiGe layer. This compressive strain is favorable for p-MOSFETs due to an increase of current drive. On the other hand, it is known that not compressive strain but tensile one can improve the characteristics of nMOSFETs (C. S. Smith, Phys. Rev., 94, 42 [1954]). Therefore, compressive strain in the SiGe layer is not desirable for the nMOSFETs. For an application to a CMOS circuit, it is useful to start with a strained SGOI substrate and then relax the channel only for nMOSFETs so as to apply tensile strain. But it is the most promising way to prepare a strain-relaxed SGOI substrate and then apply compressive/tensile strain to the channel for p/n-MOSFETs respectively by SiGe epitaxial growth. It is so hard, however, that a strain-relaxed SGOI substrate with high quality could be formed.
The second problem is that defects and dislocations in the SGOI layer should be generated by the compressive strain as the composition of Ge increases during Ge condensation. In particular, these defects and dislocations in the channel result in deterioration in mobility and junction leakage, which cause significant problems in device characteristics and reliability. Therefore, when a CMOS circuit is formed on the SGOI substrate, it is indispensable to achieve both relaxation of strain and reduction in defects in the SGOI layer.